1. Field of the Invention
The present invention relates to a digital still camera having an electronic shutter function, and more particularly, to a solid-state image capturing device for carrying out smear correction without deteriorating an image resolution according to the number of pixels of a solid-state image capturing device, a smear correcting method and the digital still camera.
2. Description of the Related Art
FIG. 9(a) is a schematic plan view showing a conventional solid-state image capturing device described in JP-A-10-136391 proposed by the applicant. In a solid-state image capturing device 10, a photoreceptor group in which a photoreceptor 11b having a blue filter attached thereto and a photoreceptor 11r having a red filter attached thereto are arranged alternately and transversely in a line and a photoreceptor group in which a photoreceptor 11g having a green filter attached thereto is arranged transversely in a line are shifted by a ½ pitch and are alternately arranged in a vertical direction.
In the drawing, the photoreceptors 11r, 11g and 11b are displayed in squares inclined by 45 degrees. In the solid-state image capturing device, a transfer electrode 16 is provided along each side of the squares of the photoreceptors 11r, 11g and 11b. A transverse line H1 of the transfer electrodes is divided by a channel stop to correspond to each photoreceptor and the same electric potential is applied to the electrode group H1 in a transverse line in the same timing.
In the conventional example, 4-phase driving is carried out. Therefore, the electrode group in a transverse line repeats H3, H4, H1 and H2, and an electrode terminal 21 is connected to the electrode group H1, an electrode terminal 22 is connected to the electrode group H2, an electrode terminal 23 is connected to the electrode group H3 and an electrode terminal 24 is connected to the electrode group H4.
The transfer electrode 16 provided around each of the photoreceptors 11r, 11g and 11b also forms a line in a vertical direction and a transfer electrode group 12 arranged in the vertical direction (only one column is shown in a dotted line) constitutes a vertical register 12. When a read potential is applied to a read gate which is not shown in order to read an image signal picked up by the solid-state image capturing device (the accepted charge of each of the photoreceptors 11r, 11g and 11b), the accepted charge (signal charge) of each of the photoreceptors 11r, 11g and 11b is read to the transfer electrode of the vertical register 12 as shown in an arrow on each of the photoreceptors 11r, 11g and 11b. 
By sequentially applying a transfer potential (a transfer pulse) to each of the electrode terminals 21 to 24, then, the accepted charge is transferred in a vertical direction (a downward direction in an example shown in the drawing) and the signal charge is transferred to a horizontal register 13 provided in a lowermost stage through a transfer electrode 17. The signal charge is transferred in a horizontal direction by applying the transfer pulse to electrode terminals 25 and 26 and is output from an output section 14 of the horizontal register 13 as shown in an arrow 27.
FIG. 9(b) is a view showing the transfer state of the vertical register 12. An encircled numeral in the drawing represents a lower figure of the reference numerals of the electrode terminals 21 to 24 (corresponding to FIGS. 1 to 4 of the electrode groups H1 to H4 in a transverse line of FIG. 9(a)). By cyclically applying a transfer potential (for example, a pulse having a high level of 0V and a low level −8V) to each electrode terminal, an electric potential well is moved along the vertical register 12 and signal charges Qi, Qi−1, . . . constituting an image signal are transferred to the horizontal register 13.
The transfer is carried out by repeating such a control that {circle around (1)} an electric potential having a high level is applied to the electrode terminal 21 to expand the electric potential well into three electrodes, {circle around (3)} the electric potential of the electrode terminal 23 is set to be an electric potential having a low level to reduce the well into two electrodes, and {circle around (2)} the electric potential of the electrode terminal 22 is set to be an electric potential having a high level to expand the electric potential well into three electrodes, . . . from the state shown in the drawing.
In the case in which an image is to be picked up at a high shutter speed by means of a digital still camera mounting a solid-state image capturing device thereon, an electronic shutter is used in place of a mechanical shutter. More specifically, when a signal charge is being transferred by the vertical register 12, the photoreceptor is set in such a state as not to be shielded by the mechanical shutter.
For this reason, a light or an electron leaks out of the photoreceptors 11r, 11g and 11b to the vertical register 12 during the transfer of a signal charge through the vertical register 12, and smear charges qi, qi−1, . . . which are the cause of smear (a bright line in a vertical direction appearing in an image when the sun is photographed) enters the electric potential well in addition to original signal charges Qi, Qi−1, . . . and are transferred together with the signal charge. When the smear charge and the signal charge of an image signal are mixed as shown in FIG. 9(b), there is a problem in that both of them cannot be distinguished from each other, resulting in a deterioration in picture quality.
If it is possible to detect the degree of the smear charge in the electric charges thus transferred, the smear can be decreased by correcting the amount of electric charges which are transferred. In the example of the transfer shown in FIG. 9(b), a progressive scanning type is used. Therefore, it is impossible to distinguish the smear charge from the signal charge. If an interlace scanning type is used, the smear charge can be distinguished from the signal charge.
FIG. 10(a) is a schematic plan view showing a solid-state image capturing device 20 of the interlace scanning type. The solid-state image capturing device 20 has different electrodes 21 to 24 and connecting structures thereof from those of the solid-state image capturing device 10 shown in FIG. 9(a). In the solid-state image capturing device 20, an electrode group H3a arranged horizontally is connected to an electrode terminal 3a, an electrode group H4 is connected to an electrode terminal 4, an electrode group H1b is connected to an electrode terminal 1b, an electrode group H2 is connected to an electrode terminal 2, an electrode group H3b is connected to an electrode terminal 3b, and an electrode group H1a is connected to an electrode terminal 1a. 
FIG. 10(b) is a view illustrating the transfer of the interlace scanning type. In the interlace scanning type, a signal charge received by a photoreceptor group is read every horizontal line and is thus transferred by the vertical register 12 so that a vacant well is transferred every horizontal line differently from progressive scanning. The signal charge does not enter the vacant well but the smear charge enters the vacant well and is thus transferred. By detecting the amounts of the smear charges qi+1 and qi−1 in the vacant well, therefore, it is possible to estimate the degree of the smear charge qi transferred together with the signal charge Qi by averaging the amounts of the smear charges qi+1 and qi−1, for example.
In the interlace scanning type, thus, the smear charge amount in the image signal can be estimated. By carrying out smear correction, therefore, it is possible to generate an image having less smear. Because of the interlace scanning, however, there is a problem in that the vertical resolution of an image is reduced to a half as compared with the number of pixels.